It is known from U.S. Pat. No. 6,092,278 to provide ignition voltage for a spark plug in an internal combustion engine by use of a high voltage step-up transformer mounted directly above the spark plug. The high voltage transformer utilizes a magnetic core having a pencil-shape, and thus has become commonly known as a “pencil core”.
Such a pencil core is shown at 10 in prior art FIG. 1 from the '278 patent and is formed of a plurality of stacked thin magnetic metal laminations 11 of varying width, but having a substantially constant thickness and a same length so that a substantially circular profile as shown in prior art FIG. 2 results.
In order to maintain the stack as a unified body, it is known in the '278 patent to provide a plurality of embossments 12A, 12B, 12C in the laminations 11 so that the embossment of an upper lamination fits into the inside of an embossment of the following lamination and so on until the last lamination at the bottom. This is most clearly shown in FIG. 2 which is a cross-section taken along line II-II in FIG. 1.
It is also known as shown in FIG. 1 to provide vertical rectangular channels 8 and 9 at end faces of the pencil core where the channels are formed from individual cutouts in each of the laminations. These channels 8 and 9 are used to keep the core vertically aligned as it proceeds through a die.
There are a number of disadvantages to such a pencil core. First, the steps 13 shown in FIG. 2, the embossments 12A, B, C, and the cutouts to form channels 8 and 9 result in a decreased electrical performance of the core. Also not as much electrical steel is provided for a given core diameter because of the steps 13.
It is known from U.S. Pat. No. 6,501,365 to provide a substantially circular pencil core 14 as shown in prior art FIG. 3. According to the '365 patent, such a circular pencil core is manufactured with the following steps.
First, the individual laminations 16 of a constant length corresponding to the length of the desired finished pencil core are cut but with varying width (such as by blanking in a stamping die). Thereafter the laminations 16 are stacked to form a stacked assembly which is clamped and subjected to a mechanical machining so that the stepped dotted portions 15 of the laminations 16 are removed. However, this mechanical machining produces burrs on the lateral portions of the laminations at the lateral edges and these burrs provide undesired electrical conductive paths between laminations which may produce electrical shorts. The mechanical machining also produces undesired smears which may also create electrical shorts.
It is thus necessary to remove the burrs and the smears, such as by electro-chemical etching, for example. The burrs and smears are indicated at 18 in FIG. 3.
Thereafter, as shown in prior art FIG. 4, the end faces 19 and 20 of the machined and de-burred/de-smeared core stack 14 are welded to form the finished pencil core.
The above manufacturing procedure has a number of disadvantages. The welds at the ends decrease the electrical performance of the core. Furthermore, the process is complicated and requires the electro-chemical etching removal of the burrs and smearing. Furthermore, during the machining process, the loose laminations of the stack must be clamped together during the machining process prior to the welding.